1. Field of the Invention
The present invention relates a photoelectric conversion apparatus, a driving method therefor, and an X-ray system having the apparatus and, more particularly, to a one- or two-dimensional photoelectric conversion apparatus capable of performing a one-to-one read operation in a facsimile apparatus, a digital copying machine, an X-ray image pickup apparatus, or the like, a driving method for the apparatus, and a system having the apparatus.
2. Related Background Art
Conventionally, as the read system of a facsimile apparatus, a digital copying machine, an X-ray image pickup apparatus, or the like, a read system using a reducing optical system and a CCD type sensor has been used. With the recent development in photoelectric conversion semiconductor materials typified by hydrogenated amorphous silicon (to be referred to as a-Si hereinafter), there has been a remarkable development in a so-called contact type sensor, which is obtained by forming photoelectric conversion elements and a signal processing unit on a substrate having a large area, and designed to read an image of an information source through a one-to-one optical system. Since a-Si can be used not only as a photoelectric conversion material but also as a thin-film field effect transistor (to be referred to as a TFT hereinafter), a photoelectric conversion semiconductor layer and a TFT semiconductor layer can be formed at the same time.
FIGS. 20A to 20C show the structures of conventional optical sensors. FIGS. 20A and 20B show the layer structures of two types of optical sensors. FIG. 20C shows a typical driving method common to these sensors. Both the sensors in FIGS. 20A and 20B are photodiode type optical sensors. The sensor in FIG. 20A is of a PIN type. The sensor in FIG. 20B is of a Schottky type. The sensor in FIG. 20A includes an insulating substrate 1, a lower electrode 2, a p-type semiconductor layer (p-layer) 3, an intrinsic semiconductor layer (i-layer) 4, an n-type semiconductor layer (n-layer) 5, and a transparent electrode 6. The sensor in FIG. 20B includes the same components as those of the sensor in FIG. 20A except for the p-layer 3. In the Schottky type sensor in FIG. 20B, a proper material is selected for the lower electrode 2 to form a Schottky barrier such that no electrons are injected from the lower electrode 2 into the i-layer 4. The arrangement in FIG. 20C includes an optical sensor 10 as a symbol representing the above optical sensor, a power supply 11, and a detection unit 12 such as a current amplifier. The direction indicated by xe2x80x9cCxe2x80x9d in the optical sensor 10 corresponds to the transparent electrode 6 side in FIGS. 20A and 20B, whereas the direction indicated by xe2x80x9cAxe2x80x9d corresponds to the lower electrode 2 side. The power supply 11 is set to apply a positive voltage to the xe2x80x9cCxe2x80x9d side opposing the xe2x80x9cAxe2x80x9d side. The operation of this arrangement will be briefly described. When light is incident from the direction indicated by the arrow in each of FIGS. 20A and 20B, and reaches the i-layer 4, the light is absorbed to generate electrons and holes. Since an electric field has been applied from the power supply 11 to the i-layer 4, the electrons pass through the xe2x80x9cCxe2x80x9d side, i.e., the n-layer 5, and move to the transparent electrode 6, while the holes move to the xe2x80x9cAxe2x80x9d side, i.e., the lower electrode 2. That is, a photocurrent flows in the optical sensor 10. When no light is incident, neither electrons nor holes are generated in the i-layer 4. In addition, the n-layer 5 serves as a hole injection inhibiting layer, and the p-layer 3 in the PIN type sensor in FIG. 20A and the Schottky barrier layer in the Schottky type sensor in FIG. 20B serve as electron injection inhibiting layers. For this reason, the holes in the transparent electrode 6 and the electrons in the lower electrode 2 cannot move, and hence no current flows. Therefore, a change in current occurs in accordance with the presence/absence of incident light. If this change is detected by the detection unit 12 in FIG. 20C, this arrangement operates as an optical sensor.
It is, however, difficult to manufacture a photoelectric conversion apparatus with a high S/N ratio at a low cost by using the above conventional optical sensor. The following are the reasons.
The first reason is that the PIN type sensor in FIG. 20A and the Schottky type sensor in FIG. 20B each require injection inhibiting layers at two portions. In the PIN type sensor in FIG. 20A, the n-layer 5 as an injection inhibiting layer needs to have both the characteristic that guides electrons to the transparent electrode 6 and the characteristic that inhibits injection of holes into the i-layer 4. Lack of one of the characteristics leads to a decrease in photocurrent, or generation of or an increase in a current (dark current) without incident light, resulting in a decrease in S/N ratio. This dark current itself is regarded as noise and includes a fluctuation called shot noise, i.e., quantum noise. Even if such a dark current is removed by the detection unit 12, quantum noise accompanying the dark current cannot be reduced. In general, in order to improve the above characteristics, the conditions for formation of the i-layer 4 and the n-layer 5 and the conditions for annealing after formation of such layers must be optimized. However, the p-layer 3 which is another injection inhibiting layer needs to have the same characteristics as those described above, although the relationship between the electrons and holes is opposite to the above relationship. The respective conditions for the p-layer 3 must also be optimized. In general, the optimal conditions for the former n-layer are not the same as those for the latter p-layer, and it is difficult to satisfy the conditions for both the layers. That is, requiring injection inhibiting layers at two portions in a single optical sensor makes it difficult to form an optical sensor with a high S/N ratio. This applies to the Schottky type sensor in FIG. 20B as well. The Schottky type sensor in FIG. 20B uses a Schottky barrier layer as one injection inhibiting layer. This layer uses the difference in work function between the lower electrode 2 and the i-layer 4. For this reason, materials for the lower electrode 2 are limited, and the localized level of the interface greatly influences the characteristics of the Schottky barrier layer, making it more difficult to satisfy the conditions for the layer. It is also reported that a thin silicon or metal oxide or nitride film having a thickness of about 100 xc3x85 is formed between the lower electrode 2 and the i-layer 4 to improve the characteristics of the Schottky barrier layer. This structure uses a tunnel effect to guide holes to the lower electrode 2 and improve the effect of inhibiting injection of electrons into the i-layer 4. The structure also uses the difference in work function, and hence materials for the lower electrode 2 are limited. Furthermore, since the structure uses opposite properties, i.e., inhibition of injection of electrons and movement of holes by means of the tunnel effect, the oxide or nitride film is limited to a very thin film having a thickness of about 100 xc3x85. Moreover, since it is difficult to control thickness and film quality, the productivity decreases.
Requiring injection inhibiting layers at two portions leads to low productivity and high cost for the following reason. These two injection inhibiting layers are important in terms of characteristics. If, therefore, a defect is produced in one of the two layers by dust or the like, characteristics necessary for an optical sensor cannot be obtained.
The second reason will be described with reference to FIG. 21. FIG. 21 shows the layer structure of a field effect transistor (TFT) made of thin semiconductor layers. A TFT is sometimes used as part of a control unit in forming a photoelectric conversion apparatus. The same reference numerals in FIG. 21 denote the same parts as in FIGS. 20A to 20C. This TFT includes a gate insulating film 7 and an upper electrode 60. A method of forming the TFT will be described in orderly sequence. A lower electrode 2 serving as a gate electrode, the gate insulating film 7, an i-layer 4, an n-layer 5, and the upper electrode 60 serving as source and drain electrodes are sequentially formed on an insulating substrate 1. The upper electrode 60 is etched to form source and drain electrodes. Thereafter, the n-layer 5 is etched to form a channel portion. The characteristics of the TFT are easily influenced by the state of the interface between the gate insulating film 7 and the i-layer 4. In general, in order to prevent contamination of the interface, these layers are continuously deposited in the same vacuum.
This layer structure poses a problem in forming a conventional optical sensor on the same substrate on which this TFT is formed, resulting in an increase in cost and a deterioration in characteristics. This is because the layer structures of the conventional sensors in FIGS. 20A and 20B are different from the layer structure of the TFT. More specifically, the PIN type sensor in FIG. 20A has the electrode/p-layer/i-layer/n-layer/electrode structure, and the Schottky type sensor in FIG. 20B has the electrode/i-layer/n-layer/electrode structure. In contrast to this, the TFT has the electrode/insulating layer/i-layer/n-layer/electrode structure. That is, these structures cannot be formed in the same process. A complication of processes results in a decrease in yielding ratio and an increase in cost. In order to share the i-layer/n-layer, an etching process is required for the gate insulating film 7 or the p-layer 3. For this reason, the p-layer 3 as an injection inhibiting layer which is an important layer of the above optical sensor and the i-layer 4 cannot be formed in the same vacuum, or the interface between the gate insulating film 7 which is important to the TFT and the i-layer 4 is contaminated upon etching of the gate insulating film, resulting in a deterioration in characteristics and a decrease in S/N ratio.
The structure, which has an oxide or nitride film formed between the lower electrode 2 and the i-layer 4 to improve the characteristics of the Schottky type sensor in FIG. 20B, is identical to the structure in FIG. 21 in terms of the order of the layers formed. However, as described above, the oxide or nitride film needs to have a thickness of about 100 xc3x85, and hence it is difficult to use it as a gate insulating film.
FIG. 22 shows the result obtained by an experiment on the yielding ratio of TFTs in relation to the gate insulating film. Below a gate insulating film thickness of 1,000 xc3x85, the yielding ratio exhibited an abrupt decrease. At a thickness of 800 xc3x85, the yielding ratio was about 30%. At a thickness of 500 xc3x85, the yielding ratio was 0%. At a thickness of 250 xc3x85, even the operation of the TFT could not be detected. This data clearly shows that it is difficult to use the oxide or nitride film of an optical sensor using a tunnel effect as the gate insulating film of a TFT which needs to insulate electrons and holes.
In addition, although not shown, a capacitive element (to be referred to as a capacitor hereinafter) having good characteristics with a small amount of leakage current, which is an element required to obtain the integral value of charges or currents, is difficult to form with the same structure as that of a conventional optical sensor. Since a capacitor is intended to store charges between two electrodes, a layer for inhibiting movement of electrons and holes is required as an intermediate layer between the electrodes. A conventional optical sensor, however, uses only a semiconductor layer between electrodes. For this reason, it is difficult to obtain an intermediate layer having good characteristics which is thermally stable and has small leakage current.
If a TFT and a capacitor, which are the important elements of a photoelectric conversion apparatus, exhibit poor matching in terms of process and characteristics, many complicated steps are required to form a system having many one- or two-dimensional optical sensors and designed to sequentially detect optical signals. For this reason, the yielding ratio is very low, posing a serious problem in forming a high-performance, multifunctional apparatus at a low cost.
It is an object of the present invention to provide a photoelectric conversion apparatus having a high S/N ratio and stable characteristics, a driving method therefor, and a system using the apparatus.
It is another object of the present invention to obtain a required image by changing the refresh voltage in accordance with the type of image.
It is still another object of the present invention to switch refresh power supplies in a refresh operation for each photoelectric conversion element by using switches so as to realize a state in which the potential of one electrode is lower than the potential of the other electrode, and a state wherein the potential of one electrode is higher than the potential of the other electrode, thereby realizing both a mode requiring a wide dynamic range in obtaining a motion image or moving image and a mode requiring a reduction in noise in obtaining a still image.
It is still another object of the present invention to provide a photoelectric conversion apparatus allowing a high yielding ratio and having stable characteristics, and a system using the apparatus.
It is still another object of the present invention to provide a photoelectric conversion apparatus which can be formed in the same process as that for a TFT, and can be manufactured at a low cost without complicating the manufacturing process, a driving method for the apparatus, and a system using the apparatus.
In order to achieve the above objects, the present invention has the following means.
The present invention includes mode switching means for performing a refresh operation for a photoelectric conversion element by switching a mode of setting a potential of one electrode of the photoelectric conversion element to be higher than a potential of the other electrode of the element, and a mode of setting the potential of one electrode of the photoelectric conversion element to be lower than the potential of the other electrode of the element.
In addition, the present invention can switch refresh power supplies of different voltages by using switching means.
The present invention is characterized in that the photoelectric conversion apparatus comprises a photoelectric conversion element formed by depositing, on an insulating substrate, a first electrode layer, a first insulating layer for inhibiting passage of carriers of a first type and carriers of a second type having a polarity opposite to that of the carriers of the first type, a photoelectric conversion semiconductor layer, an injection inhibiting layer for inhibiting the carriers of the first type from being injected into the semiconductor layer, and a second electrode layer,
refresh mode operation means having a first refresh mode of applying an electric field to each layer to set
(VrGxc2x7q) less than (VDxe2x88x92VFB)xc2x7qxe2x80x83xe2x80x83(1)
where VrG is the voltage of the first electrode layer of the photoelectric conversion element, q is the charge of a carrier of the first type, VD is the voltage of the second electrode layer, and VFB is a threshold voltage, and a second refresh mode of applying an electric field to each layer to set
(VrGxc2x7q)xe2x89xa7(VDxe2x88x92VFB)xc2x7qxe2x80x83xe2x80x83(2)
and
switch means for switching the refresh modes and causing the apparatus to operate in one of the modes.
The present invention is characterized in that a plurality of photoelectric conversion elements each identical to the photoelectric conversion element are one- or two-dimensionally arranged, all the photoelectric conversion elements are divided into a plurality of blocks, and switch elements respectively connected to the photoelectric conversion elements are operated in units of blocks to output optical signals from all the photoelectric conversion elements divided into the plurality of blocks to matrix signal lines.
The present invention is characterized in that each of intersections between the matrix signal lines is constituted by a multilayered structure in which at least a first electrode layer, an insulating layer, a semiconductor layer, and a second electrode layer are sequentially stacked, and the respective layers of the multilayered structure are formed from the same layers as those of a first electrode layer, an insulating layer, a photoelectric conversion semiconductor layer, and a second electrode layer of the photoelectric conversion element, and have the same thicknesses as those of the respective layers of the photoelectric conversion element.
The present invention is characterized in that the apparatus is driven to obtain a desired photoelectric conversion signal by arbitrarily changing a refresh voltage for the photoelectric conversion element.
The present invention is characterized in that switching is performed to set the first refresh mode of applying an electric field to each layer to set
(VrGxc2x7q) less than (VDxe2x88x92VFB)xc2x7qxe2x80x83xe2x80x83(1)
when a still image is to be obtained, and switching is performed to set the second refresh mode of applying an electric field to each layer to set
(VrGxc2x7q)xe2x89xa7(VDxe2x88x92VFB)xc2x7qxe2x80x83xe2x80x83(2)
when a motion image is to be obtained.
The present invention is characterized in that an X-ray image pickup apparatus comprising:
a photoelectric conversion apparatus having a refresh mode;
a phosphor formed on the photoelectric conversion apparatus;
an X-ray source for generating optical information by irradiating X-rays onto the phosphor; and
changing means for changing the refresh voltage in accordance with an image.
The present invention further comprises:
a phosphor formed on the photoelectric conversion apparatus;
an X-ray source for generating optical information by irradiating X-rays onto the phosphor;
signal processing means for processing a signal from the photoelectric conversion apparatus;
recording means for recording a signal from the signal processing means;
display means for displaying the signal from the signal processing means; and
transmission means for transmitting the signal from the signal processing means.
According to the photoelectric conversion apparatus of the present invention, refresh power supplies can be switched by using switches in a refresh operation for each photoelectric conversion element so as to realize a state in which the potential of one electrode is lower than the potential of the other electrode, and a state wherein the potential of one electrode is higher than the potential of the other electrode, thereby realizing both a mode requiring a wide dynamic range in obtaining a motion image and a mode requiring a reduction in noise in obtaining a still image. In the present invention, a required image can be obtained by changing the refresh voltage in accordance with the type of image.
In addition, according to the present invention, a photoelectric conversion apparatus allowing a high yielding ratio and having stable characteristics, and a system using the apparatus can be provided.
Furthermore, since each photoelectric conversion element of the present invention can be formed in the same process as that for a TFT, each element can be manufactured at a low cost without complicating the manufacturing process.